After entering 2 1 century, especially after China's entry into WTO, domestic products are facing great challenges. All walks of life, especially traditional industries, urgently need to apply electronic technology and automatic control technology for transformation and upgrading. For example, in the textile industry, temperature and humidity are important factors affecting the quality of textiles, but the methods of temperature and humidity measurement and control in textile enterprises are still very rough and backward. Most of them still use wet-bulb hygrometers and dry-bulb hygrometers, and manually observe and adjust valves and fans. The control effect can be imagined. This is basically the case in the pharmaceutical industry. In the food industry, based on experience, few people use humidity sensors. It is worth mentioning that with the development of agriculture to industrialization, many farmers realize that it is necessary to get rid of backward traditional farming and breeding methods and adopt modern science and technology to meet the challenge of imported agricultural products and enter foreign markets. More and more greenhouses have been built around the country to plant anti-season vegetables and flowers; It is more and more urgent for aquaculture to monitor the environment; A large number of temperature-regulating cold storage buildings provide a broad market for temperature and humidity measurement and control technology. China has introduced more than 40 advanced large-scale greenhouses from the Netherlands, Israel and other countries, with high degree of automation and high cost. China is gradually digesting and absorbing related technologies, generally starting with temperature regulation, lighting regulation and ventilation control; The second step is automatic temperature and humidity control and CO2 measurement and control. In addition, a large number of national grain reserve projects put forward requirements for temperature and humidity measurement and control technology.

However, at present, in the field of humidity testing, the performance of most humidity sensors can only be used in the usual temperature environment. Most domestic humidity sensors, including many foreign humidity sensors, will frown in applications that need to measure humidity in special environments! For example, the above textile printing and dyeing industry, food industry, high temperature resistant material industry, etc. It is necessary to measure humidity at high temperature. In general, the spindle drying temperature in printing and dyeing industry can reach 120 degrees Celsius or above; In the food industry, the baking temperature of food can reach about 80-200 degrees Celsius; High temperature resistant materials, such as ceramics, can be filtered and dried to above 200 degrees Celsius. In these cases, ordinary humidity sensors are difficult to measure.

Polymer capacitive humidity sensors are usually made on insulating substrates made of glass, ceramics, silicon and other materials by screen printing or vacuum coating, and then humidity-sensitive glue is coated on electrodes by dipping or other methods to make capacitive elements. In the atmospheric environment with different relative humidity, the capacitance of the humidity sensor changes regularly because the humidity sensitive film absorbs water molecules, which is the basic mechanism of the humidity sensor. The temperature characteristics of polymer capacitor elements are affected by temperature, not only the dielectric constant ε of polymer as medium and the dielectric constant ε of adsorbed water molecules, but also the geometric size of the elements is affected by the thermal expansion coefficient. According to Debye's theory, the dielectric constant ε of liquid is a dimensionless constant related to temperature and frequency. The ε of water molecule is 78.36 at t = 5℃ and 79.63 at t = 20℃. The relationship between organic matter ε and temperature varies from material to material, and does not completely follow the proportional relationship. In some temperature regions, ε increases with the increase of T, while in some temperature regions, ε decreases with the increase of T.. In the analysis of humidity-sensitive mechanism of polymer humidity-sensitive capacitive elements, most documents think that the dielectric constant of polymer is small, for example, the dielectric constant of polyimide is 3.0-3.8 at low humidity. While that dielectric constant of wat molecules is ten of times that of polymer ε. Therefore, the dielectric constant of the water-absorbing heterogeneous layer after moisture absorption is greatly improved due to the dipole distance of water molecules, which is determined by the additivity of the composite dielectric constant of multiphase media. Due to the change of ε, the capacitance c of humidity-sensitive capacitive element is proportional to the relative humidity. It is difficult to establish the full humidity range linearity of humidity sensing characteristics in the design and manufacturing process. As a capacitor, the thickness d of polymer dielectric film and the effective area s of flat capacitor are also related to temperature. The change of medium geometry caused by temperature change will affect C value. The average thermal expansion coefficient of polymers can reach orders of magnitude. For example, the average thermal expansion coefficient of nitrocellulose is 108x 10-5/℃. With the increase of temperature, the thickness d of dielectric film increases, which has a negative contribution to c; However, the expansion of the humidity-sensitive film increases the adsorption of water by the medium, which is a positive contribution to C. It can be seen that the temperature characteristics of the humidity-sensitive capacitor are dominated by many factors, and the temperature drift is different in different humidity ranges. It has different temperature coefficients in different temperature regions; Different humidity-sensitive materials have different temperature characteristics. In a word, the temperature coefficient of polymer humidity sensor is not a constant, but a variable. Therefore, in general, the sensor manufacturer can linearize the sensor in the range of-10-60 degrees Celsius to reduce the influence of temperature on the humidity sensor.

The high-quality products of foreign manufacturers mainly use polyamide resin. The product structure is summarized as follows: vacuum evaporation of gold-plated electrodes on borosilicate glass or sapphire substrate, then spraying a planar humidity-sensitive film in the form of humidity-sensitive dielectric material (as mentioned above), and then evaporation of gold-plated electrodes on the film. The capacitance of the humidity sensor is proportional to the relative humidity, and the linearity is about 2%. Although the humidity measurement performance is ok, the temperature resistance and corrosion resistance are not ideal. In the industrial field, the service life, temperature resistance, stability and corrosion resistance need to be further improved.

Ceramic humidity sensor is a new type of sensor developed vigorously in recent years. The advantages are high temperature resistance, humidity lag, fast response speed, small volume and convenience for mass production. However, due to the porous material, which has a great influence on dust and frequent daily maintenance, it often needs to be cleaned by electric heating, which easily affects the product quality and humidity. The poor linearity in low humidity and high temperature environment, especially the short service life and poor long-term reliability, is an urgent problem for this kind of humidity sensor.

At present, in the development and research of humidity sensor, resistance humidity sensor should be the most suitable for humidity control. Its representative product, lithium chloride humidity sensor, has many important advantages such as stability, temperature resistance and long service life. Lithium chloride humidity sensor has more than 50 years of production and research history, and there are many product types and manufacturing methods, all of which apply the advantages of lithium chloride humidity sensitive liquid, especially the strongest stability.

Lithium chloride humidity sensitive device belongs to electrolyte humidity sensitive material. Among many humidity-sensitive materials, lithium chloride electrolyte humidity-sensitive liquid first attracted people's attention and was used to manufacture humidity-sensitive devices. The equivalent conductance of lithium chloride electrolyte humidity-sensitive liquid decreases with the increase of solution concentration. Electrolyte is dissolved in water to reduce the water vapor pressure on the water surface.

The substrate structure of lithium chloride humidity sensor is divided into columnar and dressing-like, and the humidity-sensitive liquid and gold electrode with lithium chloride polyvinyl alcohol coating as the main components are the three components of lithium chloride humidity sensor. Over the years, product manufacturing has been continuously improved and product performance has been continuously improved. The unique long-term stability of lithium chloride humidity sensor is irreplaceable by other humidity sensitive materials, and it is also the most important performance of humidity sensor. In the process of product production, the preparation of humidity-sensitive mixture and strict control of process are the keys to maintain and exert this characteristic.

In China, Jiuchunjian Technology relies on the National Institute of Metrology, Institute of Automation of Chinese Academy of Sciences, Institute of Chemical Engineering and other large scientific research units to engage in the research and production of temperature and humidity sensor products. Choose lithium chloride humidity-sensitive materials as the main direction to produce lithium chloride humidity-sensitive sensors and related transmitters, automatic instruments and other products. While absorbing the successful experience of this technology at home and abroad, we strive to overcome the weaknesses of traditional products and make substantial progress. The product uses Al2O3 and SiO2 ceramic substrates as substrates, which greatly reduces the substrate area, and adopts special technology to greatly improve the moisture resistance and adhesion. Five dressing electrodes made of 9% industrial pure gold were sintered on the substrate by sintering process. The humidity-sensitive mixed solution of lithium chloride is mixed with new product additives and inherent components. After special aging and coating process, the service life and long-term stability of humidity-sensitive substrate have been greatly improved, especially the temperature resistance has reached -40℃- 120℃. The unique process of combining various humidity sensors is adopted. The humidity sensitivity range of the sensor is 1%RH-98%RH, and the measurement performance is below 15%RH. Both the drift curve and the humidity sensing curve have reached a good linearization level, which makes humidity compensation easy to realize and easy to ensure the humidity measurement accuracy in a wide temperature range. The closed system of circulating cooling device is adopted, and the measured gas is sampled first, and then cooled to ensure the absolute humidity is constant, so that the temperature tolerance range of the probe is increased to about 600℃, which greatly enhances the humidity measurement function at high temperature. Successfully solved the problem of "high temperature and high humidity measurement" in the field of humidity measurement. At present, JCJ200W, a separate high-temperature temperature and humidity sensor, which can directly measure the environmental humidity in the range of 150 degrees without using any devices, has been successfully applied to wood drying, high-low temperature test chambers and other systems. At the same time, JCJ200Y products can withstand high temperatures as high as 600 degrees, and have been successfully applied to spindle automatic drying system, food automatic baking system, special ceramic material automatic drying system, export large-scale drying machinery, etc. Good results have been achieved, which fills the gap of high temperature and high humidity measurement in the field of automatic control in China and lays a certain foundation for the industrialization process in China.

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Hall element is a kind of magnetic sensor based on Hall effect, which has developed into a variety of magnetic sensor products and has been widely used. This paper briefly introduces its working principle, product characteristics and typical applications.

Hall element has the advantages of firm structure, small volume, light weight, long service life, convenient installation, low power consumption, high frequency (up to 1MHZ), vibration resistance, and no fear of pollution or corrosion by dust, oil, water vapor and salt spray.

Hall linear device has high precision and good linearity; Hall switch device has no contact, no wear, clear output waveform, no jitter, no rebound and high position repetition accuracy (up to micron level). Hall element has many compensation and protection measures, and its working temperature range is wide, which can reach -55℃ ~ 150℃.

According to the function of Hall element, it can be divided into Hall linear device and Hall switch device. The former outputs analog quantity, while the latter outputs digital quantity.

According to the nature of the detected object, its application can be divided into direct application and indirect application. The former is to directly detect the magnetic field or magnetic characteristics of the detected object, while the latter is to detect the artificially set magnetic field on the detected object and take this magnetic field as the carrier of the detected information. Through it, many non-electric and non-magnetic physical quantities such as force, moment, pressure, stress, position, displacement, speed, acceleration, angle, angular velocity, rotational speed, time when the working state changes are converted into electric quantities for detection and detection.

Working principle of hall element

Under the action of magnetic field, a transverse potential difference is generated on the metal sheet with current, as shown in figure 1:

This voltage is proportional to the magnetic field and the control current:

VH=K╳|H╳IC|

Where VH is the Hall voltage, H is the magnetic field, ic is the control current, and K is the Hall coefficient.

Hall effect is more obvious in semiconductors than in metals, so Hall elements are generally made of semiconductor materials.

Using Hall element, non-contact current measurement can be carried out. As we all know, when current passes through a long straight wire, a magnetic field will be generated around the wire. The magnitude of the magnetic field is proportional to the current flowing through the wire. This magnetic field can be collected by soft magnetic materials and then detected by Hall elements. Because the magnetic field has a good linear relationship with the output of the Hall element, the signal measured by the Hall element can directly reflect the current, that is, I∞B∞VH.

Where I is the current passing through the wire, B is the magnetic field generated by the current passing through the wire, and VH is the Hall voltage generated by the Hall element in the magnetic field B, which can be expressed as an equation when the appropriate proportional coefficient is selected. Hall sensor is made according to this working principle.

Application of Two Hall Sensors

1 hall proximity sensor and proximity switch

A permanent magnet is biased behind the Hall element, and they and the corresponding processing circuit are installed in a housing to make a probe. The input lead of the Hall element and the output lead of the processing circuit are connected by cables to form a proximity sensor, as shown in figure 1. See figure 19 for its functional blocks. (a) is a Hall linear proximity sensor, and (b) is a Hall proximity switch.

Figure 1 hall proximity sensor outline drawing

A) hall linear proximity sensor

(b) hall proximity switch

Fig. 2 Functional block diagram of Hall proximity sensor

Hall linear proximity sensor is mainly used for automatic counting of ferrous metal, thickness detection, distance detection, gear tooth counting, speed detection, speed measurement and speed regulation, gap sensing, tension detection, evenness detection, electromagnetic quantity detection, angle detection and so on.

Hall proximity switch is mainly used in various automatic control devices to complete the required position control, machining size control, automatic counting, various counting, automatic connection of various processes, liquid level control, speed detection and so on. Hall vane switch

Hall vane switch is a product that uses interrupt mode. Its appearance is shown in Figure 20, and its internal structure and working principle are shown in Figure 2 1.

Fig. 3 Outline drawing of Hall blade switch

2 Hall gear sensor

As shown in fig. 4, the new generation Hall gear speed sensor is widely used in the new generation of automobile intelligent engines, as the speed sensor of ignition timing and ABS (Anti-lock Brake System).

In ABS, the vehicle speed sensor is a very important component. The working principle diagram of ABS is shown in Figure 23. 1 in the figure is the speed sensor; 2 is a pressure regulator; 3 is the controller. During the braking process, the controller 3 continuously receives and processes the pulse signal corresponding to the wheel speed from the speed sensor 1 to obtain the slip rate and deceleration signal of the vehicle, and sends an instruction to the brake pressure regulator 2 timely and accurately according to its control logic, and the regulator responds timely and accurately, so that the brake air chamber can execute the instruction of inflation, maintenance or deflation, and adjust the brake pressure of the brake, so as to prevent the wheel from locking, realize anti-skid tail flick, and improve the braking safety and braking process. In this system, Hall sensor, as a wheel speed sensor, is a real-time speed collector in the braking process and one of the key components in ABS.

In the new generation intelligent engine of automobile, Hall gear sensor is used to detect the position of crankshaft and the movement speed of piston in cylinder, thus providing more accurate ignition time, which is difficult to be replaced by other speed sensors. It has many new advantages as follows.

(1) has high phase accuracy and can meet the requirement of 0.4 crank angle without phase compensation.

(2) The flameout detection requirement of 0.05 crank angle can be met.

(3) The output is a rectangular wave, and its amplitude has nothing to do with the vehicle speed. Further sensor signal conditioning in the electronic control unit will reduce costs.

With the gear sensor, not only the rotational speed can be detected, but also the angle, angular velocity, flow rate, flow rate, rotation direction and so on can be measured.

Fig. 4 Internal structure of Hall speed sensor

1. Wheel speed sensor 2. Pressure regulator 3. electronic controller

2. Figure 4 shows the working principle of ABS pneumatic braking system.

3 rotation sensor

As shown in fig. 5, magnets are arranged in various ways, and by combining them with a Hall switch circuit, various rotation sensors can be formed. After the Hall circuit is energized, every time the magnet passes through the Hall circuit, it outputs a voltage pulse.

(a) radial pole (b) axial pole (c) blocking type

Fig. 5 magnet arrangement of rotation sensor

Therefore, physical quantities such as revolution, rotation speed, angle and angular velocity can be detected for rotating objects. Impeller and magnet are fixed on the rotating shaft, and the impeller is driven to rotate by fluid (gas and liquid) to form a speed and flow sensor. Speedometer, odometer, etc. It can be realized by installing a magnet on the axle and installing a Hall switch circuit near the magnet. Examples of these applications are shown in fig. 25.

An impeller with a magnet is installed in the housing of fig. 6, and the Hall switch circuit is installed next to the magnet. The measured fluid is introduced from one end of the pipeline to drive the impeller to drive the magnet connected with it to rotate. When passing through the Hall element, the circuit outputs pulse voltage, and the flow rate of fluid can be obtained from the pulse number. If the inner diameter of the pipeline is known, the flow can be obtained from the velocity and diameter. The Hall circuit is powered and output by cable 35.

Fig. 6 Hall flowmeter

As can be seen from Figure 7, after simple signal conversion, the digitally displayed vehicle speed can be obtained.

Using the locked Hall circuit, not only the rotating speed can be detected, but also the rotating direction can be identified, as shown in Figure 27.

The curve 1 corresponds to the structure diagram (a), the curve 2 corresponds to the structure diagram (b) and the curve 3 corresponds to the structure diagram (c).

Fig. 7 Block diagram of Hall speedometer

Fig. 8 Use Hall switch lock to measure direction and speed.

Application of 4 in high current detection

In metallurgical, chemical, superconducting applications and high-energy physics (such as controlled nuclear fusion) test devices, there are many ultra-large current consuming devices. Using the current sensor made of multi-Hall probe to measure and control large current can not only meet the requirements of accurate measurement, but also save the expensive testing device like Rogowski coil method. Fig. 9 shows a Hall current sensor device used in a D III-D tokamak. Using this Hall current sensor, the current up to 300kA can be detected.

Fig. 9(a) shows the mounting structure of G- 10, in which the current bus is at the center, (b) a cable-type multi-Hall probe, and (c) a Hall voltage amplifier circuit.

(a)G？ 10 mounting structure (b) cable type multi-hall probe (c) hall voltage amplifier circuit

Fig. 9 Multi-Hall probe high current sensor

Figure 10 Hall clamp digital ammeter circuit schematic diagram

Figure 1 1 hall power meter schematic diagram

(1) Hall control circuit

(b) Hall magnetic field circuit

Figure 12 Hall Multiplier in Hall Three-phase Power Transmitter

Figure 13 Functional Block Diagram of Hall Watt-hour Meter

Figure 14 Functional Block Diagram of Hall Isolation Amplifier

5 Hall displacement sensor

If the working current of the Hall element remains constant and it moves in a uniform gradient magnetic field, then the value of the Hall voltage VH it outputs only depends on its displacement z in the magnetic field. Figure 15 shows the output characteristic curves of three kinds of magnetic systems that generate gradient magnetic fields and displacement sensors composed of them and Hall elements. Hall micro-displacement sensor can be formed by fixing it on the tested system. As can be seen from the curve, the structure (B) is on the Z axis.

Figure 15 Static characteristics of several magnetic systems generating gradient magnetic fields and several Hall displacement sensors.

Measuring displacement with Hall element has many advantages: small inertia, fast frequency response, reliable operation and long service life.

Based on micro-displacement detection, Hall sensors such as pressure, stress, strain, mechanical vibration, acceleration, weight and weighing can be formed.

6 Hall pressure sensor

Hall pressure sensor is composed of elastic element, magnetic system and Hall element, as shown in figure 16. In Figure 16, the elastic element of (a) is a bellows, (b) is a spring leaf, and (c) is a bellows. The magnetic system is preferably a composite system capable of forming a uniform gradient magnetic field as shown in (a) and (b) of fig. 29, or a single magnet as shown in (c). After the pressure is applied, the relative displacement between the magnetic system and the Hall element changes the magnetic field acting on the Hall element, thus changing its output voltage VH. The value of the measured pressure p can be obtained from the pre-calibrated p ~ f (VH) curve.

Figure 16 Composition Principle of Several Hall Pressure Sensors

7 Hall acceleration sensor

Figure 17 is the structural principle and static characteristic curve of Hall acceleration sensor. The homogeneous spring plate S is fixed at the O point of the box, the inertia block M is installed at the middle U of the plate S, the Hall element H for measuring displacement is fixed at the end B of the plate S, and a pair of permanent magnets are installed above and below the Hall element H, which are oppositely installed with the same polarity. The box is fixed on the measured object. When they accelerate vertically with the measured object, the inertia block causes the Hall element H to displace relative to the box under the action of inertia force, resulting in the change of Hall voltage VH. Acceleration can be obtained from the relationship curve between VH and acceleration.

Figure 17 Structure and Static Characteristics of Hall Acceleration Sensor

Triangle

At present, Hall sensors have developed from discrete components to integrated circuits, which have attracted more and more attention and been widely used.